Today, more and more grid connected street lighting is replaced by solar powered off-grid street lighting, also referred to as solar powered OSL. Present solar powered street poles use batteries to store the energy for use in the night. The most frequently used battery type for this purposes is a lead battery (i.e. Pb battery), because of its relatively low cost. Present Pb batteries have a relative short life of 2 to 3 years. This is mainly caused by the solar application: when a Pb battery would be recharged immediately after discharge, and with the correct charging profile, it may enjoy life of up to 5 years. But the solar application does not always produce enough current for a complete recharge, especially in the winter where the Pb battery is run in a partially charged condition (i.e. partial State Of Charge). The result is that Pb is typically short lived at 2 to 3 years which is not deemed sufficient. Battery lifetime is considered a main differentiator.
Alternatively, lithium batteries may be used which have much longer life, with claims of 20 years under optimal conditions. But also for Li-Ion, the outdoor solar charging conditions limit life, this time caused by the temperatures under operation. The batteries' performance under cold conditions is a fraction of the performance under warmer test conditions of typically 25° C. This is caused by the Arrhenius factor, which describes the limited chemical kinetics at lower temperatures. In addition, when using Lead acid batteries, at temperatures slightly below zero the electrolyte will freeze up. The mitigation is to over dimension the battery, so it can still release enough power.
Some battery technologies will degrade when charged at subzero temperatures. An example is the above mentioned Li-Ion technology, which will suffer from Lithium plating under such conditions when it is charged with a high current, resulting in a very strong reduction in life. Mitigation against Lithium plating may be an internal heating system, which consumes energy that had to be put into the battery. This causes increased cost for the larger PV requirement. Alternatively the battery may be buried under ground below the frost layer, at e.g. 1 m. But again, this will add cost for ground works. Another mitigation is to limit the charge current when the battery is cold, but since the charge duration is limited to the daytime and it is almost impossible to plan charging interruptions due to clouds and shadows, the battery may not be fully charged.
The document WO2011122476 (A1) describes a device which is less likely to turn off as a result of restricting the power consumed depending on the amount of power generated or stored during winter time. Disclosed is an illumination device provided with a solar cell device, a storage battery which is charged by means of the power supplied from the solar cell device, an illuminating unit which emits light by means of the power supplied from the storage battery, a sensor which measures the outer temperature, and a control device which controls the charge and discharge of the storage battery. LED lights are used in the illumination unit for which the brightness increases as the temperature drops. So when the temperature drops, the illuminating unit will need less power from the storage battery in order to maintain the same illumination level. In this way energy can be saved at low temperatures. Saving energy is to be praised but there is a risk that the temperature of the batteries will reach such low levels that the life time of the batteries is disadvantageously affected.